Method of preparing substantially iron-free ammonium sulfate from spent pickle liquor



Patented F eb. 5, 1952 ME KHQD F BREEARING SUBSTANTIALLY "IRDNeFREE AMMONIUM SULFATE FROM vPlCK-LE LIQU R Herman .13; M ssn r, W mw and Richard W- Hyd on, s. ess enmfe by mes assignments, to Republic Steel Corporation,

- Clieveland tlhio, a corporation of New Jersey .Nopltawing. -Application.January ,9, 1947, V I ,StialhNo. 721 152 I 1 The present invention relates "generally to the art of synthesizing compounds of iron" and of ammonia, and is particularly concerned with a novel method for preparing, from coke oven gas and spent pickle liquor, iron compoundssubstantially free from ammonium sulfate and high purity ammonium sulfate substantially uncon taminated by iron or iron compounds.

In the conventional practice for pickling iron and steel bars, sheets and-othershapes, wherein a'hot sulfuric acid bath is employed, the pickling bath is reused with addition of more acid as re" quired, until it becomes unsatisfactory for fur ther use because it is too saturated with salts. or too heavily loaded with dirt and sediment. This spent liquor from continuousv picklers typically contains 18 to 22% ferrous. sulfate and 8 to 12% sulfuricacid, while that frombatchpicklers typically contains 20 to 3.0% ferrous-sulfate plus 1 to 3% sulfuric acid, byprocess'es now com? monly used commercially. .Alsdpresent in the liquor are traces of' sulfates of other metals..associated with the. pickled ironor steel asalloying elements or impurities. This, spent pickling liquor must. ;:bedisposed of..: Many attempts have; been madelpeconflmije cally to recover :the chemical values in. pickle liquor. In the processes developed: to data-costs of recovery have enerally exceeded. the value of the p se v red. The usualprwedure has therefore involved disposal, "most inexpensiv mann r cons sten wi h s an ards of h l s ta io and tream p ti n b a s in h er a z-l c ta .As th sestan a co m r rig d. he older prac e o dumping into strearnsv has'in many instances been abolished, and somealternativeproeedure such as neutralizing with lime and; dumping the res'ultant precipitate has been adopted. Practically all these procedures have the objective of getting rid of the pickle liquor as; expeditiously as possible, rather than the :conservationof any ofits values. An economic process for accomplishing the latter objective has long been desired.

. By virtue of the present invention, which volves not only the useful employment of spent pickle liquor but also beneficiationof coke oven gases, the foregoing. demand-is metand an economical process for simultaneously disposing of spent pickl liquo a d c nv rt ngitsch mical valu s int useful. fo m. is available, This. invention has the addedadvantage thatthc method thereof is readily controllable to. produce the desired results, coke. oven gas employed is freed e. w b P 3 e eia rlt eees lfiie .8 came. (or. 213-419 and ammonia whichare converted into valuable substances, and coke oven'gas and spent pickle liquor of a, wide variety of compositions may he treated with the same results in routine manner.

Briefly stated, the process of our present invention comprises the step of contacting a suitable ammonia-containing gas with a spent pickle liquor until the pH of the resulting solid-liquid mixture is between about 8.1 and about 8.5 and thereby precipitating substantially all the chemically' combined iron present in said liquor.

In carrying out this invention, coke oven gas or other similarly effective ammonia-containing gas, is suitably contacted with the waste pickle liquor. This might be done countercurrently, for example, in absorption towers packed with widespaced wooden grids to minimize plugging hazards due to precipitate formation. Operations should be so adjusted that the reacted pickle liquor, also called the black slurry product because oflits typicalcolor and'its solid-liquid cone stitution, leaves the system with a minimum dissolved iron content, which corresponds to a pI-l in the range between about 8.1 and about 8.5, and preferably at apH of about 8.3. Only under these conditions can a black slurry product be made from which the separation of the-precipitated iron salts by filtration, centrifuging or settling can'be readily attained while simultaneously leaving behind a solution containing am monium sulfate relatively uncontaminated with dissolved iron, sulfide or cyanide compounds. Moreover, if sufiicient gas-liquor contact is provided. th s necessary c nt l o th .e qu an be accomplish .Whil a ini extensive mova o ammo ia rom th cok o e gas, e her ,withmcs .Q h cyeni e an a least art o the H25.

T PI1P1I$1R15311 .Q 1110 from the a k liquor eparat n mu b handled s arately. The precipitated solids are washed to recover entrained ammonium sulfate solution, and can then be sintered preparatery to being charged to a blastiurnace, or processed for re-v covery of cyanide or other values, or used in the removal of iurther amounts, of H33 from the deammoniated coke oven gas, otherwise disposed of. On the other hand, theclear liquor from the black slurry separation can, if desired, be exposed to the air for a while to oxidize the small amounteof dissolved iron present to the ferric -sta.te, whereupon the ferric hydroxide formed can beremovcd by filtration or otherwise treated. Another optional stepJwouldinvolve adding sul- Iuricacid to convert the small amount of dissolved free ammonia present to sulfate, since this ammonia would otherwise be lost in the subsequent evaporation. This final evaporation step can be carried out in any suitable equipment, and the ammonium sulfate crystal product recovered. The bottoms liquor from the evaporators can be recycled to the absorption towers along with the fresh pickle liquor feed.

The importance of correct control of the black slurry composition is illustrated by the following experiment on coke oven gas, which typically analyzes 1% NHs, 0.7% H28, 0.1% HCN, 2.3% CO2 and the balance chiefly CO, H2, CH4, and illuminants. Upon bubbling such a gas through pickle liquor, located for example in a laboratory absorption bottle, at least three major stages in the absorption can be detected. During the first stage, no precipitate forms due to the presence of free acid, and the ratio of dissolved iron to sulfate in solution remains constant. The second stage shows the formation of blue-black precipitate of iron compounds, and so is characterized by a falling ratio of dissolved iron to sulfate in solution as more and more gas passes through the solution. The critical or optimum point is finally reached, at which the amount of dissolved iron,

and similarly the ratio of dissolved iron to sul fate, is a minimum. In the third stage, the amount of iron in solution increases due to a re dissolving of the precipitated iron compounds, and so this third stage is characterized by an in creasing ratio of dissolved iron to sulfate in solution.

The reactions during these three stages are quite complex. In stage one, probably little C02, HCN or HzS dissolve, since these gases are relatively insoluble in acid solutions, and since they cannot form precipitates with iron in the presence of acid. The chief reaction occurring here, therefore, is probably the neutralization of the free acid in the pickle liquor with the absorbed ammonia. In the second stage, iron precipitates due to the reaction:

Iron precipitation due to the formation of sulfides, carbonates, and cyanides also occurs, as is shown by the following typical analysis of an (unwashed) precipitate removed from a solution at the critical point between stages two and three of the absorption:

Per cent Loss at 105 C 72.98 Sulfide sulfur 9.65 Sulfate sulfur 2.16 Total nitrogen (Kjeldahl) 3.36 Total ammonia 2.88 Free ammonia 0.34 Cyanide (by difierence) 1.83 Carbon dioxide 0.05

The iron sulfides, cyanides and carbonates are all relatively insoluble at this critical point at the end of stage two, for the liquid phase now shows a minimum amount of these contaminants in solution, and therefore -chiefly contains dissolved ammonium sulfate and a little free ammonia. During stage three, however, the increasing alkalinity of the solution due to the absorption of ammonia results in further absorption of hydrogen sulfide and cyanide. Simultaneously, the precipitated iron compounds still present begin to go back into solution, probably due to the formation of soluble iron salts such as iron thiocyanide, ammonium ferrocyanide. etc.

T liquor.

The important finding here is the necessity of operating so that the pickle liquor is brought to the critical point at the end of stage two. If an excess of gas is treated, so that stage three is entered upon, then the solution is contaminated with sulfides, cyanides, and redissolved iron compounds. If an insufiiciency of gas is passed through the pickle liquor so that the critical point at the end of stage two is not reached, then the iron in the pickle liquor is not entirely precipitated. In either case, the solution of ammonium sulfate is contaminated with dissolved iron and other compounds, which makes the recovery of pure ammonium sulfate crystals by evaporation very difilcult.

The iron precipitate form-ed is extremely fine, and of a somewhat slimy, elatinous nature. It appears to improve somewhat in filterability with increasing pH. At the critical pH of 8.3,. this precipitate has been found to separate readily from the liquid phase in a high speed centrifuge. Filtration is somewhat. more difiicult, but can be carried out successfully, especially if a little filter aid is used.

The control of absorption operations, so that the black slurry produced will be at the desired critical point at the end of stage two, ma be accomplished in several ways. A most convenient procedure was found to involve following the course of the reaction by determining the hydrogen ion concentration of the pickle liquor during absorption with a standard pH meter. Fresh pickle liquor has a variable hydrogen ion concentration depending on its composition, but typically might have a pH about 2.0. As absorption proceeds, the pH increases until it is about 8.3 which is the critical point. If now the absorption is allowed to proceed further, the final pH reached is perhaps 10. It was found that the pH range between about 8.1 and about 8.5 could be used as an accurate indication that the critical point had been reached, regardless of the ratio of ferrous sulfate to free acid in the original pickle liquor or the concentration of these components in the This critical pH value also varied little with such typical changes in the analysis of the coke oven gases as might be encountered in in dustrial coke plant operation. Temperature of absorption likewise appeared to have little effect on the critical pH value. I

Control of absorption might also be achieved by any means for determining the ratio of iron to sulfate in solution in the liquid phase of the black slurry, as for example by chemical analysis. Adequate control could also be accomplished by operating to obtain a minimum concentration of dissolved iron, which might be determined by color or other suitable procedure.

The operation of this process on a larger scale might advantageously involve a continuous countercurrent contacting of the pickle liquor and the coke oven gas. This contacting might be accomplished in absorption towers packed with wooden slats, stone, Raschig rings, or similar materials, or it might be carried out in a conventional spray or bubble cap tower. The countercurrent operation of a series of stage contacts, such as the saturators now installed extensively in coke plants for ammonia recovery, would also be suitable. In other words, any orthodox gas-liquor contacting device which will not plug up with precipitate, can be used.

In the event that towers are used, the pickle liquor wouldordinarily flow down countercurrent to the ascending gas stream. The relative rates of feed of gas. and liquor to the towers would be so adjusted that the black slurry product would be at the critical point between stages two and threev of the reaction described above, or in other words, contain a minimum iron content and so have a pH between about 8.1 and about 8.5. If in actual operation, for example, a tower system was found to produce 'a. product of too low pH (under-reacted) then it is merely necessary to decrease the rate of liquor flow relative to the gas flow to improv the pH. of the product to the proper value; If, on the contrary, too high a pH value is evident, then an increase of liquor flow relative to the gas will correct this difiiculty.

It is evident that, if suflicient gas-liquid con.- tact is supplied, an extensive recovery of am monia from the gas becomes possible even while producing a black slurry product of the desired pH. It was,'for example, found possible to recover 97% of the NI-Is in a synthetic coke oven gas when operating with a two-stage gas-liquor contact, with countercurrent flow. Simultaneously, the hydrogen cyanide present. is also extensively removed from the gas by this scrubbing operation (80 to 90%) along with a part of the hydrogen sulfide present (-40%). Such removal is desirable, since the toxicity of HCN makes it a hazardous component in any fuel, especially for domestic use, while res has further disadvantages as a fuel gas component if used for steel making and the like.

The fact that the critical pH is largely independent of the concentration and the ratio of acid to ferrous sulfate the pickle liquor, as discussed above, is advantageous from an operating point of view in two ways. First, the waste pickle strength and proportions as produced in a steel plant may vary considerably depending upon a number of factors. is rare that a steel plant produces pickle liquor and coke oven gases in correct proportions for absorption, and any deficit in pickle liquor is most easily made up by sulfuric acid additions. As proposed here, operation with pH control, or control by critical concentration of iron in solution, is feasible even though the pickle liquor undergoes large variations in concentration and analysis.

The iron precipitate can be separated from the black liquor slurry by centrifuging or otherwise. This precipitate must then be washed to recover the entrained ammonium sulfate, and this wash-' ing can be done on the centrifuge or filter used, or by repulping, or similar orthodox procedures. The wash water can then be evaporated to recover ammonium sulfate. The precipitated iron salts, after washing, typically analyze on a wet basis about 75% water, 7% sulfide sulfur, somewhat under 7% CN, and the remainder chiefly iron. This analysis will of course vary somewhat, depending upon the composition of the liquor and gas used. As already indicated, these solids can now be put to one or another of various uses, e. g. dried and sintered for blast furnace use, or treated to recover cyanide or other values, etc.

The liquor left after separating the iron precipitate from the black slurry contains a small amount of ferrous iron in solution. The evaporation of this liquor without further treatment yields ammonium sulfate of good purity. In a run using typical gas and pickle liquor, it was found that ammonium sulfate containing only 0.09% iron by weight. could be made in' this Second, it

fashion. .If desired howeten tbis purity can be further improved e. g. to 0.004% by weight by first oxidizing the liquor by exposure to air or oxygen. The ferric hydroxide which forms can be separated by filtration, settling, or other methods. The suspension canalso be sent direct to the evaporators without previous solids separation,, since it'appears that the ammonium sulfate crystals formed here by evaporation are relatively uncontaminated by the ferric hydroxide floc. providing too great a percentage of the water present is not evaporated err. The bottoms liquor containing the ferric hydroxide floc is then recycled to the absorption towers along with the fresh pickle liqu'or feed.

The desirability of neutralizing the free ammonia in the liquor before sending it to the evaporators has been mentioned. If'this is not done,

this; free ammonia will be lost, or must somehow be recovered from the vapors in'the evaporators.

Having thus described the present invention so that others-maybe able to understand and practice the same; we state that what we desire to secure by Letters Patent is defined in what is claimed. i

What is claimed is:

1. The method of precipitating as ferrous compounds substantially all thechemical'ly combined iron of a spent pickle liquor which comprises the steps of contacting said liquor with a crude coke oven gas containing ammonia to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous compounds, and. "separating the solid phase from the liquid phase which consists essentially of ammoniumsu'lfate and contains substantially no dissolved iron. 2. The method of precipitating as ferrous compounds substantially all the chemically combined iron of a spent pickle liquor which comprises the steps of continuously bringing a crude coke oven gas containing ammonia into contact with said liquor to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting said liquor into a black slurry containing precipitated ferrous iron compounds, and separating the solid phase from. the liquid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron.

3. The method of precipitating as ferrous compounds substantially all the chemically combined iron of a spent pickle liquor which comprises the steps of continuously flowing said liquor in countercurrent contact with a stream of a crude coke oven gas containing ammonia to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous iron compounds, and separating the solid phase from the liquid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron.

4. The method of producing from a spent pickle liquor ferrous iron compounds substantially uncontaminated by sulfates which comprises the steps of contacting said liquor with a crude coke oven gas containing ammonia to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous iron compounds, separating the solid phase from the liquid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron, and washing the solid phase material with water substantially free of sulfate.

5. The method of preparing substantially ironfree ammonium sulfate from a spent pickle liquor which comprisesthe steps of contacting said liquor with a crude coke oven gas containing am monia to elevate and maintain the pH of said liquor between about 8.1 and 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous iron compounds, separating the solid phase from the liquid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron, contacting said liquid phase with an oxidizing gas and oxidizing any residual dissolved iron, and separatingthe ammonium sulfate from the resultingpreoipitate of oxidized iron.

6; The method of preparing substantially ironfree ammonium'sulfate from a spent pickle liquor which comprises the steps of contacting said liquor with a crude coke oven gas containing ammonia to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous iron compounds, separating the solid phase from the liquid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron, contacting said liquid phase with an oxidizing gas and oxidizing any residual dissolved iron, partially evaporating the liquid phase and crystallizing the ammonium sulfate therein, and removing the ammonium sulfate crystals from the mother liquor containing oxidized iron in solid form and separating the ammonium sulfate from the resulting precipitate of oxidized iron.

'7. The method of preparing substantially ironfree, crystalline ammonium sulfate of high purity from a spent pickle liquor which comprises the steps of contacting said liquor with a crude coke oven gas containing ammonia to elevate and maintain the pH 'of said liquor between about 8.1 and about 8.5 and thereby converting the said ill liquor into a black slurry containing precipitated ferrous iron compounds, separating the solid phase which consists essentially of ammonium sulfate and contains substantially no dissolved iron, contacting said liquid phase with air and thereby oxidizing any residual dissolved iron, separating iron thus oxidized and precipitated from the liquid phase, and crystallizing the ammonium sulfate from the liquid phase by evaporating a portion of said liquid phase.

8. The method of continuously preparing from a spent pickle liquor substantially iron-free, crystalline, high purity ammonium sulfate which comprises the steps of continuously flowing said liquor in countercurrent contact with a crude coke oven gas containing ammonia to elevate and maintain the pH of said liquor between about 8.1 and about 8.5 and thereby converting the said liquor into a black slurry containing precipitated ferrous iron compounds, continuously withdrawing portions of the liquor from contact with the gas as the pH of these portions reaches a prede termined value between about 8.1 and about 8.5, separating the solid phase of these said portions from the liquid phase thereof which consists essentially of ammonium sulfate and contains substantially no dissolved iron, and continuously evaporating said liquid phase of these portions and crystallizing ammonium sulfate therefrom.

HERMAN P. MEISSNER.

RICHARD W. HYDE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

5. THE METHOD OF PREPARING SUBSTANTIALLY IRONFREE AMMONIUM SULFATE FROM A SPENT PICKLE LIQUOR WHICH COMPRISES THE STEPS OF CONTACTING SAID LIQUOR WITH A CRUDE COKE OVEN GAS CONTAINING AMMONIA TO ELEVATE AND MAINTAIN THE PH OF SAID LIQUOR BETWEEN ABOUT 8.1 AND 8.5 AND THEREBY CONVERTING THE SAID LIQUOR INTO A BLACK SLURRY CONTAINING PRECIPITATED FERROUS IRON COMPOUNDS, SEPARATING THE SOLID PHASE FROM THE LIQUID PHASE WHICH CONSISTS ESSENTIALLY OF AMMONIUM SULFATE AND CONTAINS SUBSTANTIALLY NO DISSOLVED IRON, CONTACTING SAID LIQUID PHASE WITH AN OXIDIZING GAS AND OXIDIZING ANY RESIDUAL DISSOLVED IRON, AND SEPARATING THE AMMONIUM SULFATE FROM THE RESULTING PRECIPITATE OF OXIDIZED IRON. 